Pharmaceuticals and other biologically active molecules generally must cross biological barriers in order to be effective. For example, drugs or probes which are intended to interact with intracellular targets must cross the plasma membrane of the cell in order to produce the desired effect. Certain drugs may need to traverse other biological barriers, such as the stratum corneum of the skin for topically-applied agents. Since biological barriers such as the plasma membrane are generally composed primarily of non-polar components, the biologically active molecule must be relatively non-polar in order to be able to traverse the barrier by passive diffusion. However, the drug or probe must also be sufficiently polar in order to be soluble in blood plasma or extracellular fluid. These opposing requirements result in a relatively narrow range of polarity for biological agents and probes or reporter molecules which are intended to act within the cell, or which must traverse a biological membrane or other biological barrier to exert an effect.
Various solutions to this problem are described in U.S. Pat. No. 6,306,993, U.S. Pat. No. 6,495,663, U.S. Pat. No. 6,593,292, U.S. Pat. No. 6,669,951, U.S. Pat. No. 6,730,293 and U.S. Pat. No. 6,759,387, and in Rothbard et al., Nat. Med. 6:1253 (2000); Kirschberg et al., Org. Lett. 5:3459 (2003); Samuel et al., Proc. Natl. Acad. Sci. U.S.A. 100:14281 (2003); Chen et al., Chem. Biol. 8:1123 (2001); Kim et al., J. Immunol. 159:1666 (1997); Robbins et al., BioTechniques 33:190, 194 (2002); Siprashvili et al., Hum. Gene Ther. 14:1225 (2003), and the various articles appearing in Advanced Drug Delivery Reviews (2005), volume 57, particularly those from pages 487 to 665. These publications describe delivery-enhancing transporters; that is, transporter molecules which enhance the ability of a biologically active molecule to cross a biological barrier, such as the cell membrane or the stratum corneum.
It is desirable to have an efficient system for producing conjugates of cargo molecules, such as biologically active molecules, with transporter molecules. The conjugate of the cargo with the transporter molecule is desirably effected by a linker which is stable under conditions of storage and administration, but which releases the cargo within the cell or other target environment. It is also desirable to have a rapid, relevant and economical system for screening transporter molecules for their ability to transport biologically active molecules across a biological membrane or other biological barrier. Embodiments of the current invention addresses these objectives, and provide additional advantages, as well as providing for various compositions and methods.